'Death Spiral' Around a Black Hole Yields Tantalizing Evidence of an Event Horizon

January 11, 2001: The Hubble telescope may have, for the first time, provided direct evidence for
the existence of black holes by observing how matter disappears when it falls
beyond the "event horizon," the boundary between a black hole and the outside
universe. Astronomers found their evidence by watching the fading and disappearance
of pulses of ultraviolet light from clumps of hot gas swirling around a massive,
compact object called Cygnus XR-1. This activity suggests that the hot gas fell
into a black hole.

Q & A: Understanding the Discovery

1.
What is an event horizon, and how do astronomers know it exists?

An event horizon is the mysterious region surrounding a black hole that forever traps light and matter straying nearby. No astronomical object other than a black hole can possess an event horizon. Black holes have been inferred by observing the furious whirlpool motion of trapped gas and estimating how much mass is crammed into the tiny region of space the black hole occupies.

Previous X-ray observations have offered evidence for an event horizon by surveying black hole candidates that seem to be swallowing nearly 100 times as much energy as they radiate. Those results imply that trillion-degree gas is falling over the brink of an event horizon, like water over the edge of a waterfall. Until this Hubble observation, no telescope had ever detected what actually happens to a piece of matter swirling into the event horizon, like water down a drain.

2.
Did the Hubble telescope see an event horizon?

Hubble didn't see the event horizon  it is too small and too far away. The telescope did, however, measure chaotic fluctuations in ultraviolet light from seething gas trapped in orbit around the black hole. Hubble found two examples of a so-called "dying pulse train," the rapidly decaying, sequential flashes of light from a hot blob of gas spiraling into the black hole.

This signature matches theories of what scientists would predict to see when matter is falling so close to the event horizon that its light rapidly dims as it is stretched by gravity to ever-longer wavelengths.